The incorporation of fluorescent compounds into coating compositions and the like, to provide a nondestructive method for inspection, has been an area of rapid development in the last decade. (See, for example, U.S. Pat. No. 5,418,855; U.S. Pat. No. 5,310,604; and D. C. Neckers and J. C. Song, ACS Polymer Material Science, Eng, 71,69, 1994.) The use of ultraviolet radiation to produce cured coatings and adhesives has also grown in the same period. (See "Chemistry and Technology of UV and EB Formulation for Coatings, Inks and Paints," Vol. 1-5, PKT Oldring Editor; SITA Technology Ltd., London, England, 1991, ISBN 0947798 21 8.)
The prior art describes the incorporation of fluorescing agents into UV-cured conformal coatings as a means for identifying the presence of the cured film and for ensuring that the part has been properly covered with the coatings. (See J. Plardoff, J. Protect Coatings Linings, Vol. 9, No. 12, p.7, 1992.) The rapidly increasing industrial use of UV coating has created requirements for on-line, nondestructive measurement, as well as off-line monitoring of cure and depth of cure. Significant emphasis has been directed to the development of optical scanners which can examine documents or packages to which are applied or with which are associated UV cured inks or coatings containing fluorescent compounds, e.g., for detecting counterfeit currency, for bar code identification, etc. Coatings and inks that exhibit increased levels of response to scanner beams serve of course to enhance the effective sensitivity of the scanner, in turn enabling such electro-optical devices to handle more documents at higher speeds and with increased accuracy. It also follows that coatings containing increased levels of fluorescing agents can be reduced in thickness without diminishing the response to irradiation. In addition, the use of thinner coatings will permit excess heat generated by surface mounts or integrated circuits to be dissipated more readily.
Unlike solvent-based lacquers, which require no cure mechanism to form a film, curing of UV-based systems depends upon the correct wavelength of light striking the photoinitiator(s) employed, so as to generate the free radicals by which polymerization of the ingredients (monomers) is effected, and thereby to form the desired polymeric film. In the present state of the art, many of the fluorescing agents that are used for inspection purposes (e.g., substituted oxazole compounds, and fluoranthene), and like applications, absorb radiation in substantially the same region of the spectrum as that in which the photoinitiators employed react to generate the required free radicals. The resultant filtering or blocking phenomenon has limited the concentration of fluorescing agent that can be incorporated into a coating, ink or adhesive formulation, as the case may be, since an excessive amount of the agent will preclude adequate reaction of the ingredients and adequate depth of cure. This factor has in turn impeded the acceptance of UV technology for systems that demand a bright fluorescent response, e.g., for the production of conformal coatings.
More particularly, levels of 0.02 to 0.04 percent of the fluorescing agent can generally be incorporated into compositions containing conventional UV-curing photoinitiators without significant detriment to the depth of cure achieved. When the level of fluorescent material is increased however, such as to improve the brightness of response, it is often found that the coating will not cure properly; thus, even a nominally cured coating of only 1 to 3 mils thickness may have a soft undercoat of wet, uncured material. Exposure to radiation in both the ultraviolet and also the visible spectral regions can have the additional effect of decomposing the fluorescent agent molecule, thereby further diminishing the response of the coating to "black" light irradiation.
Use of photoinitiators which respond to the visible part of the spectrum (i.e. red shift) are one method to obviate the filtering effect of the fluorescing agent. Generally, however, they impart a red or dark yellow color to the resulting film, ink, or adhesive, and hence are undesirable from that standpoint.
The prior art describes the use of mono and bisphosphine oxides as photoinitiators, which can provide excellent depth of cure in titanium oxide-containing UV-curable coatings. The success of these phosphine oxides is attributed to their ability to respond in the near-UV/visible spectral region, and to photo-bleach. (See K. Dietliker et al, Proceeding, Rad Tech International, Vol 2., p. 693, (1994)). Certain morpholinophenyl derivatives (e.g., Irgacure 369) and titanium based photoinitiators (e.g., Irgacure 784 DC) are also available, which absorb in the visible region. (Irgacure products are commercially available from the Ciba Geigy Company.)
Acrylate formulations are well known in the art for use as adhesives, potting compounds, conformal coatings, inks, and the like. In addition to including polymerizable acrylate monomers, such formulations typically include elastomeric fillers (e.g., urethane oligomers, preferably capped to provide sites of unsaturation for enhanced reactivity), adhesion promoters in the form of organic acids (e.g., acrylic and methacrylic), inert fillers, supplemental adhesion promoters (e.g., silanes), leveling agents, and other ingredients. Reaction in formulations of this kind is normally initiated by use of a free-radical, active-oxygen catalyst (i.e., a peroxide, a hydroperoxide, or a perester), activated thermally, chemically (e.g., with an amine/aldehyde adduct and transition metal accelerator), aerobically, anaerobically, etc.; they may additionally or alternatively include a photoinitiator that is responsive to actinic radiation.
Illustrative of the prior art that is germane to the acrylate formulations hereinabove referred to are the following Bachmann and Bachmann et al United States patents, all of which are of common assignment herewith to Dymax Corporation of Torrington, Conn.: U.S. Pat. No. 4,348,503, issued Sep. 7, 1982, U.S. Pat. No. 4,429,088, issued Jan. 31, 1984, U.S. Pat. No. 4,432,829, issued Feb. 21, 1984, U.S. Pat. No. 4,963,220, issued Oct. 16, 1990, U.S. Pat. No. 4,974,938, issued Oct. 23, 1990, and U.S. Pat. No. 5,039,715, issued Aug. 13, 1991.